Question

How would you rank the following elements in the order of decreasing atomic size: $Ba,Mg\ \text{and Sr}$. And why would you choose that order?

Answer 1

Atomic size is defined as the distance between the center of nucleus of an atom and its outermost shell. Where an atom consists of a nucleus which is made up of proton and neutron and electrons are revolving around the nucleus.

Complete answer:
The order of atomic size can be explained on the basis of periodic table where periodic table is the table in which all elements are placed according to their atomic number and their reoccurrence properties and this table consists of 18 groups and 7 periods.
Now we know that atomic size decreases as we move from left to right in the periodic table this can be explained on the basis of effective nuclear charge and atomic size increases as we move down the group this is due to the increase in the number of shells.
$Ba$ is element of $2^{nd}$ group and $6^{6h}$ period, $Mg$is also element of $2^{nd}$ group but the period is different it belongs to $3^{rd}$ period of periodic table whereas $\text{Sr}$ is also an element of $2^{nd}$ group and belongs to $5^{th}$ period so on the basis of above discussion that atomic size increases as we move down the group the decreasing order can be written as
$Ba>Sr>Mg$.

Note: Barium when alloyed with aluminum then it is used to remove unwanted gases from vacuum tubes, strontium is used as a glass for color televisions in the form of cathode ray and magnesium is used in alloys and also as an aerospace construction metal.